Abstract: A formula of the self-rotating kinetic moment is derived theoretically for the self-cleaning and rotating steel spiral with plastic coat. The moment is proportional to d (spiral wire diameter), sin2α (α: the helical angle), D² (D: the spiral O.D), u² (u: the flow velocity) and ρ (fluid density). The results of the formula agree with experimental results. Therefore, the formula can serve as the theoretical foundation in optimal design of the steel spiral with plastic coat. The spiral is thick with coat but the experiment results show that the flow resistance is still in the range of the general industrial limitation.